That is mostly dependent on the vehicle design and tire design, most US cars are made to be optimized for the 55-65 MPH limits, and most care manufactures put on lower speed rated tires as well knowing that speed limit... With proper gearing and sufficient horse power an engine doesn't have to work any harder to go 90 MPH vs 65 MPH, it's all in the design, and high speed tires will last at those higher speeds as well, it's not like you will be driving those speeds every day all day...
I like laws like those in Germany that leave many rural roads with no posted limits with a law that translates to this...
"Any person driving a vehicle may only drive so fast that the car is under control. Speeds must be adapted to the road, traffic, visibility and weather conditions as well as the personal skills and characteristics of the vehicle and load."
Basically use common sense...